Steel pipelines to be laid on the bottom of the sea cannot be pre-formed or pre-adapted to the contour of the sea floor. This is because of the laying or installation procedure that must be used.
There may be tolerated a certain degree of unevenness over which the pipeline is capable of spanning or bending, provided the specific load does not produce excessively high tensile stresses in the steel. If tensile stresses exceed allowable limits, the pipeline could be deformed permanently, either by buckling or cold bending, or both. Should cold bending occur, it could propagate uncontrollably. Requirements set by classification societies for construction and operation of offshore pipelines permit a certain degree of cold bending provided that this takes place under controlled conditions. Parameters for such controlled conditions entail that a pipeline may be cold bent to a minimum radius equating to 2% strain, which is 10 times the value allowed for elastic conditions.
Pipelines to be laid on an uneven sea floor are subjected to free span because of the excessive rigidity of the pipeline. If the steel material in the pipeline wall is strained within elastic limits, a stretching of 0.2% is usually allowable. Specifications used for offshore pipelines, however, permit plastic deformation of up to 2.0% extension provided positive measures are taken to ensure that this value is not exceeded once yielding has commenced. By allowing plastic deformation, it is possible to reduce to a considerable degree the occurrence of free spanning. Deformation may be achieved by overloading the pipeline with respect to its bending capacity, by increasing the weight.
Marine pipelines having a diameter of more than 12 inches usually require a concrete coating to achieve negative buoyancy, which is necessary if the pipeline is to be submerged and also maintain a stable state with respect to the sea current. Plastic deformation of a pipeline having a weight coating of concrete will cause the concrete to crack and break loose.